An ionic compound has been used for an ion conductor for various kinds of battery cells based on ion conduction and has been employed for electrochemical devices such as primary batteries and batteries having charge/discharge mechanism, e.g., lithium (ion) secondary batteries and fuel cells, and also electrolytic capacitors, electric double layer capacitors, lithium ion capacitors, solar cells, electrochromic display devices, etc. In general, these electrochemical devices are each composed of a pair of electrodes and an ion conductor formed between the electrodes.
Examples of the ion conductor are electrolyte solutions and solid electrolytes and those obtained by dissolving an electrolyte in an organic solvent or a polymer compound or their mixture are used as the ion conductor. In the ion conductor, the electrolyte is dissolved and dissociated into a cation and an anion to exhibit ion conductivity. A battery using such an ion conductor has been used for portable electronic appliances such as lap-top type and palmtop type computers, mobile phones, video cameras, etc., and along with wide spread of these appliances, the necessity of lightweight and powerful batteries has been increased. Further, in terms of environmental issues, the importance of development of secondary batteries with longer lives has been increased.
As an ionic compound to be used for the above-mentioned secondary batteries or the like, lithium hexafluorophosphate (LiPF6) and lithium tetrafluoroborate (LiBF4), which are electrolytic salts, and cyanoborates containing alkali metals and organic cations have been proposed. An ionic compound containing the above-mentioned cyanoborate as an anionic component has a characteristic as an ionic liquid, that is, the ionic compound is a liquid even at room temperature and shows a characteristic of being thermally, physically, and also electrochemically stable and thus has been investigated for applications to various uses.
There have been proposed various methods to synthesize a compound containing tetracyanoborate (TCB:[B(CN)4]−) among the above-mentioned cyanoborates; that is, a method of reacting a compound containing boron and an alkali metal cyanide (Z. Anorg. Allg. Chem. 2000, vol. 626, p. 560-568), a method of carrying out the above-mentioned reaction in the presence of a lithium halide such as LiCl or the like (Japanese Patent Application Publication (Translation of PCT Application) No. 2006-517546), a method of reacting a boron compound such as KBF4, LiBF4, and BF3. OEt2 with trimethylsilyl cyanide (Z. Anorg. Allg. Chem. 2003, vol. 629, p 677-685, H. Willner, et al., (two others), Z. Anorg, Allg. Chem. 2003, 629, p 1229-1234, J. Alloys Compd. 2007. 427. p 61-66, R. A. Andersen, et al. (four others), JACS. 2000. 122. p 7735-7741), etc.